A pump system of the above kind is disclosed in published Dutch patent application No. 90 01 676 in the name of the present applicant. The known pump system is used for pumping a hot slurry from a supply pipe to a discharge pipe. An amount of a hot mass is drawn into the bidirectional flow pipe by means of the displacement pump with every stroke. The hot mass is forced from the bidirectional flow pipe into the discharge pipe with the subsequent delivery stroke. The displacement volume of the displacement pump and the volume of the bidirectional flow pipe are thereby geared to each other in such a manner that the amount of medium drawn in and forced out fills the bidirectional flow pipe entirely or only partially. The bidirectional flow pipe is connected to the pump chamber via a second pipe having a liquid column present therein. The movements of the pump casing are thereby transmitted to the liquid mass in the bidirectional flow pipe by the liquid column in the second pipe. The hot medium in the bidirectional flow pipe is kept separated from the pump casing by the liquid column present in the second pipe. In order to maintain the temperature at the location of the pump casing at an acceptable level, heat exchange elements are provided around the second pipe. These elements cool the medium column present in the pipe. With this known embodiment of such a pump system, the second pipe extends in a vertical direction. The advantage of this is that it enables a limited construction volume. A drawback of the vertical arrangement of said second cooled pipe is, however, that an enhanced heat transport will occur in the direction of the displacement pump as a result of conduction, and especially as a result of the occurrence of a convection current in the pipe. On the one hand, this leads to a higher temperature at the pump casing, which is no longer acceptable under certain circumstances. On the other hand, it also leads to greater heat losses, because a larger amount of heat must be removed by cooling. With slurry temperatures on the order of 150 C. these problems are still acceptable, but at higher medium temperatures, as will increasingly occur in the future, this leads to very large cooling capacities and a corresponding cooling water consumption in order to keep the temperature at the pump casing at an acceptable level. Furthermore this leads to large heat losses of the medium to be pumped, which is also disadvantageous from an energy point of view.
Another drawback of this known system is its very large construction height, which leads to very heavy concrete bases required for these systems. Furthermore, a high pressure of the medium in the supply pipe is necessary with these systems in order to overcome the pressure of the static liquid column in the vertical portion.
Another embodiment of a pump system as described above is known from EP-A-0048535. Also with this known pump system a vertical pipe is present between the bidirectional flow pipe and the pump device. The vertical pipe contains a medium column for transmitting the piston movements, whereby heat and mass transport in the direction of the pump device will again occur in this pipe as a result of conduction and convection. As a result of this, excessive temperatures will occur at the pump casing, especially with higher temperatures of the medium to be pumped, and a large amount of heat will have to be removed by cooling, which is disadvantageous from an energy point of view.
From PCT/US79/00697, a system is furthermore known wherein the inlet and outlet valves and the bidirectional flow pipe are positioned higher than the displacement pump. As a result, the direction of the convection current has a positive effect on heat transport, but the systems are only suitable for non-settling liquids, and certainly not for slurry suspensions, for which the present invention is particularly intended.
The object of the invention is to provide a pump system of the kind indicated above, wherein the aforesaid drawbacks are obviated and by means of which high-temperature media can be pumped without this leading to excessively high temperatures at the displacement pump and without having to remove an excessive amount of heat in the second pipe by cooling.